We report a direct measurement of the hydrodynamic interaction between two colloidal particles. Two micron-sized latex beads were held at varying distances in optical tweezers while their Brownian displacements were measured. In spite of the fact that fluid systems at low Reynolds number are generally considered to have no "memory," the cross-correlation function of the bead positions shows a pronounced, time-delayed anticorrelation. We show that the anticorrelations can be understood in terms of the standard Oseen tensor hydrodynamic coupling. [S0031-9007(99)08607-X]

Mixing two liquids on a microfluidic chip is notoriously hard because the small dimensions and velocities on the chip effectively prevent turbulence. We present a topological mixing scheme that exploits the laminarity of the flow to repeatedly fold the flow and exponentially increase the concentration gradients to obtain fast and efficient mixing by diffusion. It is based on helical flow channels with opposite chiralities that split, rotate, and recombine the fluid stream in a topology reminiscent of a series of Möbius bands. This geometry is realized in a simple six-stage, two-layer elastomer structure with a footprint of 400 μm×300 μm per stage that mixes two solutions efficiently at Reynolds numbers between 0.1 and 2. This represents more than an order of magnitude reduction in the size of microfluidic mixers that can be manufactured in standard multilayer soft lithography techniques.

We studied the thermal fluctuations of single DNA molecules with a novel optical tweezer based force spectroscopy technique. This technique combines femtonewton sensitivity with millisecond time resolution, surpassing the sensitivity of previous force measurements in aqueous solution with comparable bandwidth by a hundredfold. Our data resolve long-standing questions concerning internal hydrodynamics of the polymer and anisotropy in the molecular relaxation times and friction coefficients. The dynamics at high extension show interesting nonlinear behavior. PACS numbers: 87.15.Ya, 05.40.Jc, 36.20.Ey Single molecule force spectroscopy has become a valuable tool to study the intramolecular forces involved in unfolding a protein [1,2] or inducing conformational changes in a polymer [3]. These chemical forces are typically of the order of tens of piconewtons and can readily be measured with atomic force microscope techniques. Entropic forces as low as 100 fN have been measured with optical and magnetic traps [4][5][6], but thermal motion of the force-transducing latex bead in the trap limits the time resolution of these techniques and allows dynamic measurements only for piconewton force fluctuations [7,8]. We have developed a scheme using dual optical tweezer force sensors that overcomes this limitation by making a cross-correlated heterodyne measurement. The thermal motions of the two beads are independent and uncorrelated, allowing us to measure the correlations introduced by a molecule whose ends are attached to the beads. Thus, one can measure the dynamic motion of the molecule with high temporal and force resolution by computing the cross correlation between the measured forces acting on the two beads. This femtonewton force spectroscopy (FFS) scheme is general and can in principle be adapted to make high-resolution measurements of the average power stroke of molecular motors such as myosin, kinesin, and RNA polymerase. It is superior to measuring individual events [8,9] or the instantaneous cross correlation [10] since it effectively self-averages over the entire period of the measurement, giving a concomitant improvement in the signalto-noise ratio and removing human bias in separating true events from noise fluctuations.We used FFS to study the thermal fluctuations of a single DNA molecule. DNA has proved to be a useful model system to study the complex dynamic behavior of polymers, an intriguing subject of long-standing theoretical and experimental interest. An important application of polymer dynamics has been to understand the behavior of polymers in hydrodynamic flow, where one observes fascinating effects such as drag reduction in turbulent pipe flow. To help understand this problem, many years ago De Gennes predicted that polymers would exhibit a "coil stretch" phase transition in extensional flow [11]. Theoretical arguments and experiments have shown that the critical fluid strain rate at which polymers extend is ᠨ ഠ 1͞t, where t is the fundamental relaxation time of the polymer. Subtle hysteresis ...

We have studied the adsorption of poly(styrene-b-2-vinylpyridine) block copolymers from a selective solvent onto a flat solid substrate, resulting in the formation of laterally ordered microdomains. We address the question whether the lateral domain structure is due to adsorption of entire micelles from the solution. Dynamic light scattering, atomic force microscopy, surface plasmon spectroscopy, and transmission electron microscopy were used to characterize the polymer solution, the adsorption process, and the resulting polymer layer, respectively. A quantitative comparison of the results obtained with different techniques strongly indicates direct adsorption of whole micelles onto a brush formed from free copolymer chains. Furthermore, the adsorption process is found to depend strongly on the velocity at which the samples are withdrawn from the solution.

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